In organizations receiving large amounts of mail, the opening of envelopes constitutes a substantial burden. To deal with this burden, mechanical envelope openers have been used which operate by cutting a thin strip from one edge of each envelope. Such openers sometimes damage the envelope contents because of variations in envelope size and the manner in which the contents are stuffed in the envelopes. Mechanical openers also produce large volumes of paper shavings from the high speed cutting of envelopes.
It has also been proposed to open envelopes by processes involving the chemical degradation of paper, and specifically of its cellulose, at at least one edge of the envelopes, and preferably at three edges thereof, followed by mild mechanical action to remove the degraded paper.
Zacker U.S. Pat. No. 2,866,589 discloses the degradation of cellulosic paper envelopes at their edges by chemical reagents, specifically by the action of nitric acid, sodium hydroxide, or sodium hypochlorite, or by the action of sulfuric acid followed by the application of heat.
Whitman U.S. Pat. No. 3,871,573 teaches the utilization of successive applications to the edges of an envelope of a sodium alkyl sulfate and an organic acid, such as oxalic acid or acetic acid, followed by the application of heat. Gunther, Jr. U.S. Pat. No. 4,069,011 discloses a similar system, utilizing tartaric acid in combination with the sodium alkyl sulfate. These systems produce sulfuric acid in situ.
Savit U.S. Patent application Ser. No. 946,347, filed Sept. 27, 1978, and coassigned herewith, teaches that a non-noxious organic acid having at least one pK value at room temperature between about 1.5 and about 5 may be used as the sole reactant with cellulose in the presence of heat to degrade an envelope edge so that it may be opened by mild mechanical action. Tartaric acid is the preferred organic acid.
A preferred method of applying heat to an envelope edge which has been treated with a chemical agent is to bring the envelope edgeinto close proximity to a source of radiation. Radiation heating does not require contact between the heat source and the envelope edge and thus permits easier handling in high speed processes. In addition, since the intensity of radiant heating varies inversely with the square of the distance between the radiant body and the surface to be heated, radiant heating from a source close to the envelope edge heats the envelope edge to a substantially greater degree than it heats other portions of the envelope located at greater distances from the radiant source.
Since cellulosic paper is inflammable when raised to ignition temperature and since it is essential to avoid burning or degrading the cellulosic paper beyond the edge or edges which are to be opened, it is essential that the heating step be controlled within the temperature range which is high enough to effectively degrade the chemically treated edge but not so high as to degrade the untreated cellulose beyond the treated edge. Time of exposure is also a factor in a high speed envelope opening system since the envelopes in such a system do not remain in close proximity to the radiant heat source for a long enough period to reach thermal equilibrium.
In a high speed envelope opening system of the type described above it is desired to degrade the cellulose at the envelope edges and to avoid degradation of the cellulose beyond the envelope edges. Different conditions must therefore prevail at each of these locations. As described above, the primary difference between the two locations is that the chemical degrading agent has been applied only to the edges; and the chemical degrading agent makes the cellulose much more susceptible to degradation under the action of an elevated temperature.
A second difference between degradation conditions at the envelope edges and other portions of the envelope is that the envelope edges are heated to a higher temperature than other portions due to the closer proximity of the edges to the radiant heat source. This temperature difference, however, is a gradual one, rather than a sharp drop-off; and is a relatively minor temperature difference when the plane of the envelope edges is not in the immediate vicinity of the heat source. It is therefore desirable to increase the temperature differential produced by the radiant heat source as between the envelope edges and the remaining portion of the envelopes beyond the edges.
It is also desirable to increase the efficiency of heat application to the envelope edges so that the required exposure times can be shortened, resulting in higher throughputs of envelopes through the process and resulting in energy savings on a per envelope basis.